1. Field of the Invention
The present invention is directed to a process for degreening and coloring tobacco leaves and thereafter wilting, dehydrating and curing the tobacco in a shorter time. The process includes contacting the green tobacco with ethylene in a controlled atmosphere that is exchanged frequently to not only color the tobacco but also to replenish the oxygen and nitrogen content of the atmosphere and reduce the carbon dioxide produced during the process of curing.
The use of this process during the coloring stage of curing tobacco allows the temperature of the tobacco to be increased during both the wilting and dehydrating stages. Such a temperature rise during the wilting and dehydration stages decreases the time significantly to attain the final curing of the tobacco.
The curing of tobacco is an ancient art that has been practiced for centuries. With the advent of civilization the curing of tobacco became more scientific in order to achieve a more desirable color and taste sought by the discriminating user and those knowledgeable in the tobacco field.
Tobacco has been cured in environments in which the tobacco leaves are hung to dry. Under natural conditions, the time for such drying and curing to effect the desired color change would be typically many days and up to at least a week or more in some instances. Such a length of time was not acceptable leading to the determination that curing could be accelerated with the application of heat. Long ago a temperature rise in the curing environment would be brought about through the use of wood fires or kerosene burning from a wick. This procedure indeed did cut down the curing time at least several days and produced a cured out tobacco colored with a deep yellowish-orange color. This color represented tobacco that was mature and ready for use. When cured in this manner it was found that the tobacco also retained a high level of the natural oils present in tobacco.
There followed attempts to cure tobacco with other than the traditional fuels, such as wood or kerosene through the use of so-called modern curing fuels typical of which is propane. The use of propane however caused the tobacco to attain a lemon-greenish or brown color that was not the accepted mature deep yellowish-orange color forming the traditional standard. Also it was found that such tobacco cured with these fuels possessed a lower level of the natural oils than the traditionally cured tobacco. With careful research, it was learned that the loss of color and the natural oils was caused by the immediate exposure of the tobacco leaf to an artificial heat source by which the artificial heat produced a more rapid heating and drying. In contrast, the more traditional fuels that produced ethylene as a combustion product of the fuel caused a more natural and far slower curing. It become apparent that the lower curing produced a leaf that achieved a natural degradation of the chlorophyll prior to the destruction of chloroplast. It was also recognized that the ethylene in the combustion products of the traditional fuels brought about not only chlorophyll degradation but did so in such a way as to avoid destruction of the chloroplast so as to condition the tobacco leaf to respond to a subsequent faster rate of curing.
The ethylene in the tradiational fuels also assisted the enzymes necessary to increase respiration and ripening of the tobacco leaf so as to allow the leaf to hydrolize some of the materials and respire them away at a much lower temperature than would be possible if ethylene were not used. The presence of ethylene therefore enabled the leaf to respond much more quickly to the curing process than leaves not treated with ethylene as would have been the case with the curing fuels.
The result of the experience with ethylene is that it has become a well-known fact in the tobacco curing art that ethylene has a significant effect upon the curing process and does produce a leaf that is not only acceptable to the industry but more desirable than those leaves that have been artificially cured with curing fuels.
While it is always desirable to select the mature green leaves to attain quality tobacco even when ethylene is used, it has been found that, when tobacco leaves have grown under adverse weather conditions or with other environmental difficulties, such tobacco leaves will possess less than ideal quality, which ethylene can be used to improve. It has been found, for instance, that the ethylene ripening of those leaves that are not of superior quality can still produce a high quality cured tobacco leaf.
The prior art is replete with processes for providing cured tobacco. For instance, ethylene for curing tobacco has been generated by the catalytic conversion of ethyl alcohol. Such a process has been commercially successful in that it provided for the production of ethylene without the use of the large compressed gas tanks that would otherwise have supplied the necessary ethylene. However the process had the serious drawback that the ethylene was produced from ethyl alcohol that the law required to be denatured. The denaturing of the alcohol by the addition of various denaturants often produces undesirable reaction products other than ethylene. In addition, these denaturants gradually poison or otherwise degrade the active catalyst that is the basis for the reaction of ethyl alcohol into ethylene and thus produced not only less ethylene but more of the undesirable products.
The reaction products that are produced with a degraded catalytically generated ethylene can have adverse effects upon the quality of the cured tobacco and also at times make it difficult to determine what concentration is to be expected to be generated from the amount of denatured ethyl alcohol that is sought to be catalytically converted. Accordingly, it is believed that the ideal process for curing tobacco does not reside in the catalytic production of ethylene but rather in the use of ethylene even if it is in the compressed gas cylinders that have been in use in the past.
It is known through research performed by experts in the field that ethylene will induce degradation of chlorphyll and, at the same time, will accelerate the coloring process of tobacco even though this latter point is sometimes disputed because it is not clearly understood. It is also known that in addition to the chlorophyll degradation, ethylene acts as a degradation agent by increasing the transpiration of the moisture from the tobacco leaf. This of course results in the tobacco wilting and therefore allowing greater efficiency in drying due to the passage of the forced air used typically for drying.
The concentration of ethylene gas that has been used in the past varies considerably. It is known that the concentration of ethylene that is naturally released by the yellowing tobacco within a drying kiln has been observed to be 0.13-0.30 ppm. Over a 25 hour period of yellowing, the amount of ethylene released from leaf tissue is about 1.3 ppm per one thousand cubic centimeters of leaf area.
It has been reported in the literature that ethylene gas is useful to ripen fruits and vegetables when the concentration is in the range of 1 to 1,000 ppm and particularly when maintained at levels higher than that would have been present naturally. Armed with this knowledge, many in the art have believed that if a little ethylene gas would work well that a lot more would be better to produce a more significant ethylene response including faster ripening and improved color changes. Unfortunately this belief when put into practice has resulted in a number of explosions that caused serious injuries and deaths of personnel working within the curing barns. The reason for the explosion is that the level of ethylene reaches an explosive concentration when in the range of approximately 3.1%-32% by volume in air.
Safety has therefore been of primary importance in the use of ethylene. The typical catalytically produced gas was thought to be a solution to the problem of explosions. However, that expectation has not been proven to be satisfactory in practice. Gas contained in pressurized cylinders can be administered by a so-called shot method wherein the gas contained in the cylinder is quickly released or may be permitted to escape through a gas valve regulator that meters the release of ethylene over a period of time.
In any of these or other methods, the ethylene concentration is still so variable and uncontrolled that explosions do occur and the curing process is still slow not able to produce quickly the proper color of the tobacco leaf. Increasing the speed of the curing process to produce the proper color has been sought therefore is as yet an unattainable goal, particularly when an unsafe ethylene concentration must be avoided.
In another area of concern, it is known in the art that carbon dioxide, though desirable in minute quantities where it can have a beneficial effect upon the ethylene action, does at higher concentrations deleteriously affect the ethylene response. It has been determined that the presence of the carbon dioxide, directly or indirectly, competes with ethylene at sites within the plant tissue thereby retarding the degradation of the chlorophyll. The increase of carbon dioxide is naturally achieved during the ripening and curing process and unless removed or controlled will negate many of the benefits of the ethylene in the atmosphere.
In the past, it has been known to periodically open doors of curing barns or ripening rooms to prevent the buildup or increase in the concentration of carbon dioxide. This haphazar approach to the removal or lessening of the carbon dioxide level by exchanging the air at most several times a day never addressed any of the problems in a manner that produced satisfactory results.
Tobacco treated with ethylene is capable of coloring faster, but it is desired that the time for the wilting and dehydrating stages also be shorted so as to accelerate the total curing time required for the tobacco. No prior art process is known to exist that adequately accelerates the curing time.